DESCRIPTION: This project seeks to specify neuroendocrine bases for seasonal rhythms in behavior and physiology of mammals. The model species is the Siberian hamster, Phodopus sungorus. The present approach departs from most previous analyses of seasonality in rodents by maintaining hamsters under photoperiodic conditions that more accurately simulate those present in the natural habitat. The underlying premise is that photoperiodic history and pattern of change of day length are equal in importance to absolute day length in regulating seasonal responses. The development of a procedure that permits non-invasive quantitative monitoring of gonadal condition in individual animals has made possible several new analyses of seasonality in reproduction. Specific aims include: 1) assessment of the impact of photoperiodic history early in life on adult responsiveness to different day lengths, 2) documentation of the impact of different short day lengths on the interval timer that induces neuroendocrine refractoriness to photoperiod, 3) evaluation to the contribution of morning and evening circadian oscillators to gonadal involution in a simulated natural photoperiod, 4) elaboration of melatonin signal frequencies and durations necessary and sufficient for optimal gonadal regulation by photoperiod, and 5) determining whether separate hypothalamic mechanisms mediate gonadal involution in fall day lengths and spontaneous recrudescence in winter photoperiods. The proposed work is relevant to the specification of proximate causes of seasonal rhythms in human behavior, and development of rational phototherapies for treatment of seasonal mood disorders, jet lag, and sleep and eating disturbances. An understanding of seasonal timing is germane to individual survival and reproduction of mammals.